A technology for electronic key systems used in vehicles has been developed. A transmitter capable of transmitting radio waves is mounted in the trunk of a vehicle, and a mobile device responds to the radio waves. If the trunk that is unlocked is locked, the transmitter transmits radio waves to the inside of the trunk (refer to, for example, Japanese Unexamined Patent Application Publication No. 2006-77408). According to this technology, upon receiving the radio waves transmitted by the transmitter, the mobile device returns a signal including identification information. Accordingly, a control unit detects that the mobile device is locked in the trunk on the basis of reception of the response from the mobile device. Upon detecting that the mobile device is not located in the passenger compartment of the vehicle and is locked in the trunk, the control unit performs an electric process (e.g., sends an alert or opens the trunk).
The above-described existing technology has the following issue. That is, if a sufficient space for disposing the transmitter is not available in the trunk, it is difficult to directly transmit radio waves to the inside of the trunk and, thus, it is difficult to determine whether a mobile device is located in the trunk. In addition, if as in MR layout vehicles, the space under the hood serves as a trunk, it is difficult to mount a pop-up mechanism for electromagnetically opening the hood due to a structural reason. A mobile device is locked in the trunk in the following situation. That is, when all the doors of the vehicle are locked and only the trunk is open and if the user closes the trunk with the mobile device left in the trunk, such a situation occurs. At that time, there is no way for the user to open the trunk. In general, a trunk is unlocked only by operating a trunk unlock lever disposed in the passenger compartment of the vehicle, operating a button of the mobile device, or operating a switch disposed outside the trunk to communicate with a regular mobile device. Thus, once the trunk is closed, the trunk cannot be opened unless the user performs any one of the above-described operations.